1. Field of the Invention
The present invention relates to a fixing device configured to transfer a toner image from a photoconductive element to a sheet or recording medium and then fix the toner image on the sheet and an image forming apparatus and a color image forming apparatus including the same.
2. Description of the Background Art
It has been proposed to combine the function of a fixing device and the function of an image transferring device, which are included in an electrophotographic image forming apparatus, for implementing a simple configuration. Japanese Patent Laid-Open Publication No. 2000-242109, for example, discloses an image forming apparatus in which a roller, supporting a heat-resistant, image transfer body support belt, and a fixing roller, contacting the belt, each accommodate a heater therein while the belt effects image transfer and pressing at the same time. This apparatus, however, must heat the transfer body support belt for fixation and must cool off a photoconductive element in order to protect it from degeneration.
On the other hand, Japanese Patent Laid-Open Publication No. 2000-352879, for example, teaches an image forming apparatus in which an exciting coil is disposed in the loop of an intermediate image transfer belt, which turns in contact with a press roller and a photoconductive element, for heating magnetic metal forming the core of the belt by induction heating. In this case, the intermediate image transfer belt plays the role of an intermediate image transfer body and the role of a fixing belt at the same time. A problem with this apparatus is that there must be selected a material that satisfies both of the performance as an intermediate image transfer belt and the performance as a fixing belt. In addition, this apparatus is also required to heat the intermediate image transfer belt for fixation and to cool off the photoconductive element to protect it from degeneration.
Other image forming apparatuses of the type described are taught in, e.g., Japanese Patent Laid-Open Publication Nos. 9-15933, 9-114282, 10-63121, 10-307486 and 2000-275982. However, apparatuses taught in these documents, like the apparatus of Laid-Open Publication No. 2000-242109 mentioned earlier, each dispose a heater in a belt support roller and must therefore heat a belt and must cool a photoconductive element. This limits the allowable range of fixing conditions including nipping time and thereby makes it difficult to improve a margin as to fixation. Consequently, the kind of sheets applicable to the apparatus is limited while image quality is rendered unstable.
Further, when the intermediate image transfer belt plays the role of an intermediate transfer body and the role of a fixing belt at the same time, as taught in Laid-Open Publication No. 2000-352879, it is necessary to heat the intermediate transfer belt and to cool off the photoconductive element, making it difficult to maintain the surface of the belt uniform (removal of residual toner and impurities) This also makes the image quality unstable. In addition, it is likely that the entire apparatus becomes sophisticated and therefore high cost.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 2000-330397, 2001-272866 and 2002-62701.